Progress in FY2018 was in the following areas: TRIPLE SEGMENTAL LABELING OF HIV-1 CAPSID ASSEMBLIES: We have prepared HIV-1 capsid protein assemblies in which only the N-terminal domain (NTD) of the capsid protein is triply isotopically labeled (2H, 15N, and 13C). The C-terminal domain is unlabeled. We have then performed a variety of two-dimensional (2D) and three-dimensional (3D) solid state NMR measurements on tubular assemblies of the segmentally labeled capsid protein, with the goal of identifying solid state NMR signals that arise from the intermolecular NTD-CTD interfaces within these assemblies. Since most hydrogen sites in the NTD are deuterated, and since most 15N and 13C nuclei are in the NTD, signals in 2D and 3D spectra that result from spin polarization transfers from protons (i.e., non-deuterated hydrogen sites) to 15N or 13C nuclei should come from these interfaces. In our recent experiments, we do observe signals selectively from a subset of the 15N and 13C sites in the NTD. We are currently working on the interpretation of these signals. We hope to obtain new information about amino acid sidechain interactions and configurations in intermolecular interfaces that stabilize HIV-1 capsid assemblies. SEGMENTAL LABELING OF HIV-1 GAG POLYPROTEIN ASSEMBLIES: We have developed protocols for preparing a segmentally isotopically labeled HIV-1 Gag polyprotein construct (deltaMA-CA-SP1-NC, 348 amino acids total) in which the isotopically labeled segment begins near the end of the final alpha-helix of the CA subunit (helix 11 of CA) and extends throught the NC subunit. We have also developed protocols for preparing virus-like particles (VLPs) from the segmentally labeled Gag construct. We plan to perform solid state NMR measurements on these VLPs, as soon as VLP production is scaled up to the 10 milligram level. The solid state NMR measurements will provide a more detailed structural picture of the CA-SP1 junction helix bundle, which is now known to stabilize the Gag lattice of immature HIV-1, than is currently available from our previous solid state NMR measurements on fully labeled VLPs (Bayro et al., JACS 2016) and from cryoEM measurements by the Briggs group (Schur et al., Science 2016).